Seismic wavefields observed near active volcanoes contain plenty of information about volcanic
activities in the frequency range between 0.01 Hz and 1.0 Hz, where, until recently, conventional
geophysical monitoring of volcanoes had not paid much attention except for a few rare cases. The
physics operating in a volcano is extremely complex; in different frequency bands, different physical
processes dominate the wavefield, which makes broadband seismic observations essential to unravel
the physical processes of volcanic activity using seismic data.

　Since 1991, we have been conducting a series of observations in two active volcanoes in Japan:
Sakurajima and Aso. A series of deployments of broadband instruments in Sakurajima not only
convinced us of the importance of broadband seismic observations at volcanoes, but also led us to
another volcano by observing strange long period 10s wave trains apparently generated near the Aso
volcano located 150km away from Sakurajima. Deployment of a dense network of broadband
seismometers in Aso revealed a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is
continually resonating with periods as long as 15s, even when there is no apparent surface activity at
the crater. When phreatic eruptions are observed, broadband seismograms elucidate vividly the whole
eruption process: gradual pressurization and long-period (about 20s) pulsations of the reservoir during
the 100-200s before the initiation of the discharge, followed by gradual deflation of the reservoir
concurrent with the discharging flow. The observed spatial variation of the signal amplitudes reveals
that the source of 15s long period tremors (LPTs) consists of an isotropic expansion (contraction) and
an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters.
The extension of the buried crack plane meets the crater chain, including the active fumarole at the
surface, suggesting that the crack has played an important role in transporting steam (gasses) and/or
lava to the craters from below. This work also demonstrates a powerful usage of broadband
seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.

　Our results so far demonstrate the high potential for broadband seismic observation near active
volcanoes to add essential information which has been missed by conventional seismometry. There is
now much evidence from volcanoes of the world indicating the presence of long-period volcanic
signals. We feel that volcanology should embrace state-of-the-art developments, and broadband
seismometers as well as other instruments, such as infra-sonic microphones, should be installed as
standard equipment for monitoring volcanic activities.

Fig.1. Broadband seismograms (vertical component) of a phreatic eruption observed at a
   station 1.4km away from the active crater. (top) raw velocity seismogram; (middle)
   velocity seismogram bandpass filtered between 10s and 30s; (bottom) displacement
  obtained by integrating the top record. The vertical broken line indicates the time of
                                    the eruption.

Fig.2. (top) A typical long period tremor (in velocity); (middle) the inferred crack-like
     conduit; (bottom) the observed and the model-predicted amplitudes of LPT are
  respectively represented as red and blue circles whose radii are proportional to LPT
                                     amplitudes.

USU2000:

　Mt. Usu erupted at ~13:10 (JST) on March 31, 2000 after twenty-some years of quiescence (cf. 7-2).
We (with a team from Hokkaido Univ.) were able to install a broadband seismometer just 10 minutes (!)
before the eruption ~3.5km from the eruption site. Broadband seismograms of this eruption
(magma-phreatic) indicate that no precursorly slow deformation, which was observed in Aso, exists for
this eruption. On the other hand, we were able to observe 12sec long period tremors apparently
emitted from the observed pressure source region at about 4km depth (Fig. 3).

　Long period tremors first observed in Aso appear to be a common feature of many active volcanoes,
which proves importance of broadband seismometry at active volcanoes (see also 7-3 for the Miyake
eruption).